This invention relates to a focusing degree-detecting device used for an automatic focusing apparatus which detects, by using infrared rays, whether or not an optical system is in an in-focus state, and if not, automatically places the optical system in the in-focus state. More particularly, this invention relates to a focusing degree-detecting device which can precisely detect the degree of focusing in the optical system irrespective of the displacement of image planes by infrared rays and visible rays, which arises from a chromatic aberration of an objective and in accordance with the achromatic (or apochromatic) type of the objective and the focal length (magnification) of the objective.
Good response and high accuracy are required for the focusing degree-detecting devices which are used in optical instruments and medical devices such as cameras, microscopes, rigid-endoscopes, and fundus cameras. One of the prior arts of this type of device is Japanese Patent Disclosure (KOKAI) No. 60-42725 describing the light mark projection-type detecting device. In this prior art, in order to detect the degree of focusing, emitted light beam and reflected light beam are passed through different optical paths in an optical system. The shift of the reflected light spot of the light source of the image plane is detected as the focusing degree. In the projection-type detecting devices, the TTL-type detecting devices have been the predominant type used. Further, since the reflected projection beams enter the viewing field, the infrared rays are used for the projection rays.
The image planes determined by infrared rays and visible rays are displaced due to the chromatic aberration of the objective. Therefore, some measure must be taken, in order to correct this displacement of the image planes. Chromatic aberration varies according to the focal distance and the achromatic type of the objective. Therefore, each time the objective is replaced, the amount of correction of the correcting means must be altered. This is problematic particularly in the case of a microscope in which the difference in focal distances of the objectives varies greatly.
A device for correcting displacement of the image planes due to the objective replacement is disclosed in Japanese Patent Disclosure (KOKAI) No. 58-217909 (West German Patent Application No. P3219503.6). In this device, a correction optical system, movable by a fixed distance in the direction of the optical axis, is provided for moving the image plane as determined by infrared rays. When the objective is replaced, the correction optical system is moved.
The principles of this device will be described with reference to FIGS. 16 to 18. FIG. 16 illustrates how objective A forms an image. Solid lines indicate light beams of visible rays, and broken lines light beams of infrared rays. The laser beams of infrared rays, which are emitted from laser diode 101, are projected onto an object, through convex lens 102, correction lens 103, tube lens 104, and objective A. Mirror 105 is provided between convex lens 102 and correction lens 103. The laser beams reflected by mirror 105 are focused by imaging lens 106 onto photo sensor 107, so that an image of the object is formed thereon.
FIG. 17 shows how objective B, having a focal distance different from that of objective A, forms an image. In this optical system, the image formed by visible rays is formed at the same position as that in the case of FIG. 16, but the image formed by infrared rays is formed at a position different from that in the case of FIG. 16. This is due to the fact that the different focal distances of the objectives used lead to different aberrations. In this case, the infrared rays from laser diode 101 are not focused on photo sensor 107, and the optical system is thus in the out-of-focus state.
As is shown in FIG. 18, correction lens 103 is manually or electrically moved for correcting the focusing position of infrared rays.
The manual operation for such delicate and precise positioning of the objective, for each replacement of the objective, is troublesome work and requires considerable skill. Positioning by electrical means requires an expensive motor and its drive and control circuitry, so that the product is bulky in structure and increased in cost.